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core/vm: refactor memory resize #33056

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7e30dfb
core/vm: refactor memory resize
MariusVanDerWijden Oct 30, 2025
8c5a76e
core/vm: re-add doc
MariusVanDerWijden Oct 30, 2025
c51e1b5
core/vm: clear memory
MariusVanDerWijden Nov 10, 2025
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7 changes: 6 additions & 1 deletion core/vm/memory.go
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,7 @@ func (m *Memory) Free() {
// To reduce peak allocation, return only smaller memory instances to the pool.
const maxBufferSize = 16 << 10
if cap(m.store) <= maxBufferSize {
clear(m.store)
m.store = m.store[:0]
m.lastGasCost = 0
memoryPool.Put(m)
Expand Down Expand Up @@ -79,7 +80,11 @@ func (m *Memory) Set32(offset uint64, val *uint256.Int) {
// Resize resizes the memory to size
func (m *Memory) Resize(size uint64) {
if uint64(m.Len()) < size {
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@rjl493456442 rjl493456442 Nov 10, 2025

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As we always reuse the Memory instance without clearing.

// Free returns the memory to the pool.
func (m *Memory) Free() {
	// To reduce peak allocation, return only smaller memory instances to the pool.
	const maxBufferSize = 16 << 10
	if cap(m.store) <= maxBufferSize {
		m.store = m.store[:0]
		m.lastGasCost = 0
		memoryPool.Put(m)
	}
}

After the resize, the junks will also be visible, for instance:

  • T0; memory is empty (the underlying slice is 0x1,0x2)
  • T1; memory is resized to 2, the memory becomes [0x1, 0x2]
  • T2; the EVM only use the first byte somehow (not sure if it's possible), then the memory becomes [correct-data, junk]

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@rjl493456442 rjl493456442 Nov 10, 2025

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diff --git a/core/vm/memory.go b/core/vm/memory.go
index 5e11e83748..2e71c16c1a 100644
--- a/core/vm/memory.go
+++ b/core/vm/memory.go
@@ -44,6 +44,8 @@ func (m *Memory) Free() {
 	// To reduce peak allocation, return only smaller memory instances to the pool.
 	const maxBufferSize = 16 << 10
 	if cap(m.store) <= maxBufferSize {
+		clear(m.store) // zero out the bytes before truncating the length
+
 		m.store = m.store[:0]
 		m.lastGasCost = 0
 		memoryPool.Put(m)
@@ -79,7 +81,11 @@ func (m *Memory) Set32(offset uint64, val *uint256.Int) {
 // Resize resizes the memory to size
 func (m *Memory) Resize(size uint64) {
 	if uint64(m.Len()) < size {
-		m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
+		if uint64(cap(m.store)) > size {
+			m.store = m.store[:size]
+		} else {
+			m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
+		}
 	}
 }

m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
if uint64(cap(m.store)) > size {
m.store = m.store[:size]
} else {
m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
}
}
}

Expand Down
7 changes: 7 additions & 0 deletions core/vm/memory_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -83,3 +83,10 @@ func TestMemoryCopy(t *testing.T) {
}
}
}

func BenchmarkResize(b *testing.B) {
memory := NewMemory()
for i := range b.N {
memory.Resize(uint64(i))
}
}
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